Recent development of metal borate halides: Crystal chemistry and application in second-order NLO materials

Metal borate halides refer to the metal borates containing additional halogen anions (i.e., F, Cl, Br or I). In recent years, motivated by the well-known high performance nonlinear optical (NLO) material, KBe2BO3F2 (KBBF), metal borate halides have been arousing intensive interest. The combination of halogen anions with borate groups affords a large number of metal borate halides with anionic structures ranging from 0D clusters, 1D chains, 2D layers to 3D networks. In addition, the introduction of halogen anions into borates is proven to be an effective strategy to design new noncentrosymmetric (NCS) crystals, which is required for NLO materials. In this review, recently discovered metal borate halides are summarized, including their synthesis, crystal structure features, properties and second-order NLO performances. This review also presents the structure-properties relationship studies focusing on the positive influence of introducing halogen anions on the optical properties. We hope that this work will obtain a clear perspective on crystal chemistry of metal borate halides and facilitate the discovery of new second-order NLO materials for urgent demands in laser sciences.